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03.04. Implement Queue using Stacks

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Description

Implement a MyQueue class which implements a queue using two stacks.

 

Example:

MyQueue queue = new MyQueue();



queue.push(1);

queue.push(2);

queue.peek();  // return 1

queue.pop();   // return 1

queue.empty(); // return false

 

Notes:

• You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

 

Solutions

Python3

class MyQueue:
    def __init__(self):
        """
        Initialize your data structure here.
        """
        self._s1, self._s2 = [], []

    def push(self, x: int) -> None:
        """
        Push element x to the back of queue.
        """
        self._s1.append(x)

    def pop(self) -> int:
        """
        Removes the element from in front of queue and returns that element.
        """
        if len(self._s2) == 0:
            while self._s1:
                self._s2.append(self._s1.pop())
        return self._s2.pop()

    def peek(self) -> int:
        """
        Get the front element.
        """
        if len(self._s2) == 0:
            while self._s1:
                self._s2.append(self._s1.pop())
        return self._s2[-1]

    def empty(self) -> bool:
        """
        Returns whether the queue is empty.
        """
        return len(self._s1) + len(self._s2) == 0


# Your MyQueue object will be instantiated and called as such:
# obj = MyQueue()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.peek()
# param_4 = obj.empty()

Java

class MyQueue {
    private Stack<Integer> s1;
    private Stack<Integer> s2;

    /** Initialize your data structure here. */
    public MyQueue() {
        s1 = new Stack<>();
        s2 = new Stack<>();
    }

    /** Push element x to the back of queue. */
    public void push(int x) {
        s1.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        if (s2.empty()) {
            while (!s1.empty()) {
                s2.push(s1.pop());
            }
        }
        return s2.pop();
    }

    /** Get the front element. */
    public int peek() {
        if (s2.empty()) {
            while (!s1.empty()) {
                s2.push(s1.pop());
            }
        }
        return s2.peek();
    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return s1.empty() && s2.empty();
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */

Go

type MyQueue struct {
	s1, s2 []int
}

/** Initialize your data structure here. */
func Constructor() MyQueue {
	return MyQueue{
		s1: make([]int, 0),
		s2: make([]int, 0),
	}
}

/** Push element x to the back of queue. */
func (this *MyQueue) Push(x int) {
	this.s1 = append(this.s1, x)
}

/** Removes the element from in front of queue and returns that element. */
func (this *MyQueue) Pop() int {
	if len(this.s2) == 0 {
		this.transfer()
	}
	v := this.s2[len(this.s2)-1]
	this.s2 = this.s2[:len(this.s2)-1]
	return v
}

/** Get the front element. */
func (this *MyQueue) Peek() int {
	if len(this.s2) == 0 {
		this.transfer()
	}
	return this.s2[len(this.s2)-1]
}

/** Returns whether the queue is empty. */
func (this *MyQueue) Empty() bool {
	return len(this.s1) == 0 && len(this.s2) == 0
}

func (this *MyQueue) transfer() {
	for len(this.s1) > 0 {
		this.s2 = append(this.s2, this.s1[len(this.s1)-1])
		this.s1 = this.s1[:len(this.s1)-1]
	}
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * obj := Constructor();
 * obj.Push(x);
 * param_2 := obj.Pop();
 * param_3 := obj.Peek();
 * param_4 := obj.Empty();
 */

TypeScript

class MyQueue {
    private inStack: number[];
    private outStack: number[];

    constructor() {
        this.inStack = [];
        this.outStack = [];
    }

    push(x: number): void {
        this.inStack.push(x);
    }

    pop(): number {
        if (this.outStack.length === 0) {
            this.inToOut();
        }
        return this.outStack.pop() ?? -1;
    }

    peek(): number {
        if (this.outStack.length === 0) {
            this.inToOut();
        }
        return this.outStack[this.outStack.length - 1] ?? -1;
    }

    empty(): boolean {
        return this.inStack.length === 0 && this.outStack.length === 0;
    }

    inToOut() {
        while (this.inStack.length !== 0) {
            this.outStack.push(this.inStack.pop());
        }
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * var obj = new MyQueue()
 * obj.push(x)
 * var param_2 = obj.pop()
 * var param_3 = obj.peek()
 * var param_4 = obj.empty()
 */

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